1. Technical Field
The invention refers to an illuminating device for backlighting flat displays, in particular displays for mobile applications.
2. Related Prior Art
Known illuminating devices for displays for mobile applications comprise a light source, which may be a tubular light source (CCFL), for example. The light emitted from the light source is coupled into a light guiding member of wedge-shaped cross section at an end face thereof. Specifically because of the wedge-shaped configuration of the light guiding member, a total reflection of the light beams occurs at the phase boundary and the light beams emerge from the surface of the wedge-shaped light guiding member through corresponding scattering centres. The surface of the wedge-shaped light guiding member is arranged opposite the display to be transilluminated. The surface of the light guiding member is structured such that the light emerges from the light guide by refraction. The light thus refracted is collimated and directed by a plurality of films arranged between the light guiding member and the display, such that a substantially white light reaches the display. The structure of such illuminating devices is complicated, the more so, since a plurality of films have to be placed into a frame or the like, for example, and it has to be ensured that a slipping or the like of the films is avoided. Due to the complicated structure, the manufacturing costs are high. Moreover, there is a risk of operational failures.
From EP 1 194 915 a light guiding member for mobile phone displays is known. Light from a light source, such as a LED, is coupled into the light source. To couple the light out, the light guiding member has a diffractive structure on a surface thereof. Here, the entire surface is provided with different gratings such that the gratings immediately adjoin each other. The gratings are configured such that they have different diffraction efficiencies to obtain uniformity of the coupling out of the light. Different diffraction efficiencies of the gratings are achieved by the amplitude of the individual gratings or the height of the structure varying. Making such a surface structure with different grating amplitudes is extremely difficult. Further, due to the immediate contiguity of the individual gratings, it is likely that diffraction effects that are hard to predict, such as undesired interferences, for example, because of the precision technically difficult to obtain.
It is the object of the invention to provide an illuminating device for backlighting flat displays that guarantees an increased operational reliability with a high efficiency and at low cost.